Preprints
https://doi.org/10.5194/se-2020-175
https://doi.org/10.5194/se-2020-175

  19 Oct 2020

19 Oct 2020

Review status: a revised version of this preprint was accepted for the journal SE.

Kinematics of subduction in the Ibero-Armorican arc constrained by 3D microstructural analysis of garnet and pseudomorphed lawsonite porphyroblasts from Ile de Groix (Variscan belt)

Domingo Aerden1,2, Mohammad Sayab4, Aidan Forde3, and Alejandro Ruiz-Fuentes1 Domingo Aerden et al.
  • 1Departamento de Geodinámica, Universidad de Granada, Spain
  • 2Instituto Andaluz de Ciencias de la Tierra, CSIC/Universidad de Granada, Spain
  • 3Saorgus Energy Ltd, Kerry, Ireland
  • 4Geological Survey of Finland, Espoo, Finland

Abstract. The small island of Groix in southern Brittany, France, is well known for its excellent outcrops of Variscan blueschists, eclogites and garnetiferous micaschists that define a Late-Devonian suture between Gondwana and Armorica. The kinematics of polyphase deformation in these rocks is reconstructed based on 3D microstructural analysis of inclusion trails in garnet- and pseudomorphed lawsonite porphyroblasts using multiple, differently oriented thin sections of single samples and X-ray tomography. Three sets of inclusion trails striking NE-SW, NNW-SSE and WNW-ESE are interpreted to witness a succession of different crustal shortening directions orthogonal to these trends. The curvature sense of sigmoidal- and spiral-shaped inclusion trails of the youngest set is shown to be consistent with southward thrusting or northward subduction of Gondwana under Armorica, provided that these microstructures developed by overgrowth of actively forming crenulations instead of the previously envisaged 'snowball' mechanism. The latter predicts an opposite thrusting direction which is at odds with the regional tectono-metamorphic zonation in the Ibero-Armorican Arc. Strongly non-cylindrical folds locally found on Ile de Groix are reinterpreted as fold-interference structures instead of having formed by progressive shearing. Six additional samples of lower-grade footwall units of the Groix ophiolite were also studied. The oldest inclusion trails in these rocks have similar trends as the youngest one in Ile de Groix. Our new inclusion-trail data for southern Brittany bear a strong resemblance with those documented previously in the north-western Iberian Massif and suggest about 20° anticlockwise rotation of Iberia during the early Cretaceous opening of the Gulf of Biscay.

Domingo Aerden et al.

 
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Domingo Aerden et al.

Domingo Aerden et al.

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